1. Field of the Invention
The present invention relates to leak detectors and, more particularly, to a leak detector which increases the line pressure to as high a value as possible during the metering cycle of the leak detector.
2. Description of Related Art
Gasoline dispensers used at automotive service stations dispense gasoline from an underground tank through a nozzle placed in the fill tube of an automobile gas tank. The underground tank includes a pump actuated by a user upon manipulation of a lever at the time of lifting the nozzle from its stored position on the gasoline dispenser. Downstream of the pump is a conventional leak detector, which may be of a piston or diaphragm type. For example, a diaphragm leak detector long used since 1976 is known as a Red Jacket Two-Second Model No. 116-017 Leak Detector; a similar unit has been used since 1959. These units are manufactured by the Marley Pump Company.
The gasoline line downstream of the leak detector may have a leak which creates an environmental hazard to the adjacent underground and a severe safety hazard to personnel. Such leaks are difficult to detect from the surface of the ground and the leak detectors serve the function of precluding delivery of gasoline through the delivery line if a leak of greater than a predetermined rate is present. Trapped air in the delivery line usually does not present a safety hazard but it may have the effect of extending the leak sensing function of a leak detector to the irritation of a user attempting to dispense gasoline. Changes in environmental temperatures may expand or contract the gasoline (product) within the delivery line which will vary the pressure in the delivery line and have an effect upon the operation of the leak detector. Upon closing of the dispensing nozzle, a pressure wave rebounds from the dispensing nozzle upstream through the delivery line to the leak detector. The pressure wave, often referred to as hydraulic hammer, subjects the leak detector to a potential for severe damage of critical parts, including rupture of the diaphragm in a diaphragm type leak detector. Such damage may be undetected for an extended period of time. If leaks in the delivery line are present or result from the damaging effects of hydraulic hammer, the damaged leak detector would be incapable of serving the function of detecting such leaks. The resulting environmental and safety hazards would continue unchecked. Hydraulic hammer also causes the leak detector to open prematurely because of the resulting excess pressure downstream of the leak detector. Just turning on the pump, without actuating the nozzle, may cause hydraulic hammer to a detrimental extent.
A mechanical line leak detector is a pressure sensitive valve that incorporates a built-in metering device. The leak detector is placed into the line between the pump in fluid communication with an above-ground or an underground gasoline tank (storage tank) and the dispenser at the outflow end of the gasoline (product) delivery line in such a manner that the pump is on the upstream side of the leak detector and the delivery line and dispenser are on the downstream side. Most modem pumps for storage tanks have a leak detector port built in for this purpose. Simply stated, when the pump at the storage tank is energized, the leak detector will go to a slow-flow position (poppet valve completely down) for line pressurization. When sufficient line pressure has been achieved, the poppet valve will move upward to a metering position (cross-holes in the poppet valve aligned with a pin head). At this point, a predetermined amount of product is metered into the line downstream of the leak detector. If there is a leak in the line that is as great as or greater than the predetermined metered flow entering the line, the pressure in the line will not increase and the leak detector will not open for full product flow. If there is no leak, line pressure will continue to increase and the poppet valve will move to its full open position allowing full flow of product into the line. The functions recited above are true of all mechanical leak detectors known to date.
To have a trouble free leak detector, numerous problems must be addressed. These include: hydraulic hammer created in the line which may cause the leak detector to open prematurely; loss of line pressure due to a faulty check valve at the pump drawing product from the storage tank; lack of repeatedability in the metering position of the leak detector due to small tolerances that may vary over time; leak sensing or metering pressure in the line being too low to achieve the greatest sensitivity and speed of operation; thermal contraction in the line; high frequency pressure surge from the storage tank pump causing premature opening of the leak detector; premature closing of the leak detector during a fueling operation due to reduced pressure; and, excessive size and weight of prior art leak detectors rendering installation, maintenance and removal more of a problem than necessary.